Chemistry · Structure Of Atom · NEET
The work function is the minimum amount of energy an electron needs to escape from the surface of a metal. Electrons are held inside the metal by attractive forces. To pull one out, you must supply at least this much energy. It is written as W0 and is measured in joules (J) or electron volts (eV). Every metal has its own fixed value. For example, caesium has a low work function (about 1.9 eV) so it releases electrons easily, while metals like silver need more energy.
The threshold frequency is the minimum frequency of light that can just knock an electron out of a metal. Light comes in packets called photons, and each photon carries energy E = h x nu. If the frequency nu is too low, one photon does not carry enough energy, so no electron comes out. At exactly nu0, the photon energy equals the work function, and the electron is just barely freed with zero speed. NCERT gives the threshold frequency of potassium as 5.0 x 10^14 Hz.
No. They describe the SAME barrier but in two different units. Work function is that barrier measured as ENERGY (in joules or eV). Threshold frequency is that same barrier measured as a FREQUENCY (in Hz). They are connected by the equation W0 = h x nu0, where h is Planck's constant. So if you know one, you can find the other. Do not treat them as identical numbers; they are just two ways of writing the same escape condition.
The formula is W0 = h x nu0. Here W0 is the work function, h is Planck's constant (6.626 x 10^-34 J s), and nu0 is the threshold frequency. To find nu0 from W0, rearrange: nu0 = W0 / h. If the work function is given in eV, first convert it to joules by multiplying by 1.602 x 10^-19 before dividing by h.
Because one photon can only give its energy to one electron in a single collision. If the photon frequency is below nu0, its energy h x nu is less than the work function, so the electron cannot pay the full 'exit fee' and stays inside. NCERT shows this: red light shining on potassium for hours ejects no electrons, but even weak yellow light (above nu0) instantly ejects them. Making the low-frequency light brighter does not help, because brightness adds more photons, not more energy per photon.
No. The threshold frequency depends only on the metal, not on how bright the light is. Intensity means the NUMBER of photons per second, not the energy of each photon. Below nu0, no matter how intense the beam, no electrons escape. Above nu0, higher intensity only increases the NUMBER of electrons ejected, not the threshold. This is a very common NEET trap.
First get the threshold frequency using nu0 = W0 / h. Then use lambda0 = c / nu0, where c is the speed of light (3 x 10^8 m/s). A shortcut is lambda0 = hc / W0. Remember: a HIGH work function means a HIGH threshold frequency but a SHORT (small) threshold wavelength, because frequency and wavelength are inversely related.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Work function is an energy, so its SI unit is the joule (J). In physics and chemistry problems it is often given in electron volts (eV), where 1 eV = 1.602 x 10^-19 J.
Alkali metals like caesium, rubidium and potassium have low work functions, so they release electrons easily with lower-frequency light. NCERT gives the work function of caesium as about 1.9 eV.
Yes. Each metal has its own fixed threshold frequency and work function. A metal that holds its electrons tightly has a higher work function and therefore a higher threshold frequency.
At nu0 the photon energy equals the work function, so the electron just escapes with zero kinetic energy (zero speed). Any frequency higher than nu0 gives the electron extra energy that becomes its kinetic energy.
Work function and threshold frequency are the base for Einstein's photoelectric equation and stopping potential questions, which appear regularly in NEET. Getting W0 = h x nu0 right is essential before solving those numericals.
